1. Field of the Invention
The present invention relates to a variable attenuator used for the gain control of a radio-frequency signal, and particularly to a variable attenuator suitable for circuit integration applied to a wireless communication device and the like, and a wireless communication device.
2. Description of the Background Art
In recent years, mobile phones have progressed from second-generation mobile phones using a PDC system or a GSM system to third-generation mobile phones using a WCDMA system. In the WCDMA system, it is essential to control the output level of a transmitted signal from a terminal in accordance with the distance from a base station. In the current standards, it is necessary to control a transmission level of 80 dB.
An example of a wireless device using the WCDMA system will be described below. Specifically, the example is a wireless device using polar modulation. In the wireless device, a phase-modulated signal from baseband is inputted to a PLL, and the PLL controls a VCO. From the VCO, the phase-modulated signal having a frequency N times that of a carrier frequency is outputted, and the frequency is divided by N by a frequency divider. Normally, 2 or 4 is used as N. The output signal of the frequency divider is inputted to a power amplifier via a variable attenuator and an RF amplifier. To the power amplifier, an amplitude-modulated signal from baseband and an output level control signal are also inputted. In the power amplifier, amplitude modulation corresponding to the amplitude-modulated signal is superimposed on the phase-modulated signal inputted from the RF amplifier. As a result, the output signal is an amplitude-modulated and phase-modulated signal using QPSK modulation, 8PSK modulation, or the like. Further, the output level of the output signal is controlled by the power amplifier in accordance with the output level control signal. The output signal of the power amplifier is transmitted from an antenna via a duplexer.
As described above, output power is controlled by the power amplifier, but the variable range of the power amplifier is 30 to 40 dB at a maximum. Therefore, the variable attenuator is required to control an output range of 40 to 50 dB.
FIGS. 11 through 14 show examples (first through fourth examples) of conventional variable attenuators. FIG. 11 is a diagram showing the structure of a conventional variable attenuator 501 (the first example), disclosed in Japanese Patent Publication No. 3216808. Referring to FIG. 11, the conventional variable attenuator 501 uses a GaAsFET, of which the drain and the source are connected to a bias circuit 502 for temperature characteristics compensation (i.e., a temperature characteristics compensation circuit). FIG. 12 is a diagram showing the structure of a conventional variable attenuator 504 (the second example), disclosed in Japanese Patent Publication No. 3784664. Referring to FIG. 12, the conventional variable attenuator 504 controls the attenuation by changing the bias voltage in accordance with desired attenuation and also with the changes of the reflection coefficients with respect to the preceding and following circuits, with the use of a table stored in a ROM 506, a calculation circuit 505, and D/A converters 507a and 507b. 
FIG. 13 is a diagram showing the structure of a conventional variable attenuator 508 (the third example), disclosed in Japanese Laid-Open Patent Publication No. 9-46175. Referring to FIG. 13, the conventional variable attenuator 508 uses a GaAsFET, of which the gate is connected to a temperature characteristics compensation circuit 509. FIG. 14 is a diagram showing the structure of a conventional variable attenuator 510 (the fourth example), disclosed in Japanese Laid-Open Patent Publication No. 2005-244877. Referring to FIG. 14, the conventional variable attenuator 510 uses a GaAsFET, of which the drain and the source are connected to a temperature characteristics compensation circuit 511, different from the type of circuit in FIG. 11.
FIG. 15 is a diagram illustrating a problem of the conventional variable attenuators. Referring to FIG. 15, the vertical axis represents the attenuation and the horizontal axis represents the number of stages of a variable attenuator. In FIG. 15, an upper solid line represents the minimum value of the attenuation and a lower solid line represents the maximum value of the attenuation, with respect to each stage of the variable attenuator. The difference between the minimum value and the maximum value is the variable range of the attenuation. As can be seen from FIG. 15, the larger the number of stages of the variable attenuator, the wider the variable range of the attenuation.
At the same time, the larger the number of stages of the variable attenuator, the larger the minimum value of the attenuation. The attenuation of the variable attenuator becomes noise. Particularly, the attenuation value when the attenuation is minimum satisfies the specification of receiving band noise of a wireless device, and therefore cannot be increased to more than a certain value. In a conventional wireless device, the specification of the attenuation value when the attenuation is minimum is enormous, i.e., more than 10 dB, since a filter is used between an RFIC and a power amplifier. However, this interstage filter is likely to be removed so as to miniaturize a wireless device. As a result, the specification of the attenuation value when the attenuation is minimum is required to be merely a few dB.
With the use of the conventional variable attenuators, a one-stage variable attenuator can obtain a variable range of the attenuation of merely 10 to 20 dB. To satisfy the specification of the attenuation value when the attenuation is maximum, four to five stages are required, since the variable range of the attenuation is not proportional to the number of stages. In this case, however, the attenuation value when the attenuation is minimum exceeds the specification at the same time. To improve this trade-off, it is necessary to increase the attenuation of a single-stage variable attenuator.